1. Field of the Invention
The invention relates to fabrication of organic circuits such as circuits containing organic thin film transistors.
2. Discussion of the Related Art
Organic thin film transistors (TFTs) are expected to become key components of the plastic circuitry in, among other things, display drivers of portable computers and pagers, and memory elements of transaction cards and identification tags. A typical organic TFT is shown in FIG. 1. The TFT contains a source electrode 10, a drain electrode 12, a gate electrode 14, a gate dielectric 16, a substrate 18, and the semiconductor material 20. When the TFT operates in an accumulation mode, the charges injected from the source 10 into the semiconductor are mobile and conduct the source-drain channel current, mainly in a thin channel region within about 100 Angstroms of the semiconductor-dielectric interface. (See, e.g., M. A. Alam et al., "A Two-Dimensional Simulation of Organic Transistors," IEEE Transactions on Electron Devices, Vol. 44, No. 8 (1997).) In the configuration of FIG. 1, the charge need only be injected laterally from the source 10 to form the channel. In the absence of a gate field, the channel ideally has few charge carriers, and there is ideally no source-drain conduction. The off current is defined as the current flowing between the source 10 and the drain 12 when charge has not been intentionally injected into the channel by the application of a gate voltage, and for an accumulation mode TFT, this occurs for a gate-source voltage more positive (for p-channel) or negative (for n-channel) than a certain voltage known as the threshold voltage. (See, e.g., S. M. Sze, Semiconductor Devices--Physics and Technology, John Wiley & Sons (1985).) The on current is defined as the current flowing between the source 10 and the drain 12 when the channel is conducting. For a p-channel accumulation-mode TFT, this occurs at a gate-source voltage more negative than the threshold voltage, and for an n-channel accumulation mode TFT, this occurs at gate-source voltage more positive than the threshold voltage. It is desirable for this threshold voltage to be zero, or slightly positive, for n-channel operation. Switching between on and off is accomplished by the application and removal of an electric field from the gate electrode 14 across the gate dielectric 16 to the semiconductor-dielectric interface, effectively charging a capacitor.
One of the most significant factors in bringing such TFTs into commercial use is the ability to deposit organic semiconducting materials on a substrate quickly and easily (i.e., inexpensively), as compared to silicon technology, e.g., by reel-to-reel printing processes. Yet, in order to exhibit suitable electrical properties, the organic materials need to be deposited as thin, uniform, crystalline films, which is a difficult task when using easy, inexpensive processes. (See, e.g., C. Cai et al., "Self Assembly in Ultrahigh Vacuum: Growth of Organic Thin Film with a Stable In-Plane Directional Order," J. Am. Chem. Soc., Vol. 120, 8563 (1998).)
Thus, those in the art have sought to attain the necessary uniformity and order in an organic semiconductor film by the easiest possible fabrication techniques. Cai et al., supra, use an organic molecular beam deposition technique to attain an ordered film, but such a technique is not feasible for large-scale fabrication of organic circuits. Other groups have used a polytetrafluoroethylene layer as a template for an organic semiconductor layer, but the organic material is deposited by a vapor phase method which, again, is unsuitable for inexpensive commercial-scale fabrication. (See, e.g., P. Damman et al., "Morphology and NLO properties of thin films of organic compounds obtained by epitaxial growth," Optical Materials, Vol. 9, 423 (1998); and P. Lang et al., "Spectroscopic Evidence for a Substrate Dependent Orientation of Sexithiophene Thin Films Deposited onto Oriented PTFE," J. Phys. Chem. B, Vol. 101, 8204 (1997).)
As suggested in Cai et al., supra, liquid phase techniques are the most desirable for fabrication of organic TFTs, since they involve simply providing the organic material in solution, depositing the solution on a substrate, and removing the solvent. Such liquid phase techniques have been somewhat successful for polymeric semiconductor materials, as reflected, for example in Z. Bao et al., "High-Performance Plastic Transistors Fabricated by Printing Techniques," Chem. Mater., Vol. 9, 1299 (1997); and H. Sirringhaus et al., "Integrated Optoelectronic Devices Based on Conjugated Polymers," Science, Vol. 280 (1998). However, polymeric semiconductor materials tend to be highly sensitive to the fabrication conditions, are difficult to purify to the extent required for high mobilities and on/off ratios, and exhibit substantial batch-to-batch variability. Oligomeric materials and/or polymers of relatively low molecular weight, and other low molecular weight compounds, tend to be less sensitive and are generally able to be purified such that high, and less environmentally sensitive, mobilities and on/off ratios are possible. Thus, liquid phase techniques for depositing low molecular weight compounds, such as oligomeric semiconductor materials and low molecular weight polymeric materials, are of particular interest.
For example, several groups have experimented with solution casting of thiophene oligomer films, in which a solution of the organic material is essentially dropped onto a substrate, and the solvent is evaporated by heating. However, these processes have generally provided relatively poor uniformity and coverage from such oligomeric solution casting. Relatively large areas, e.g., greater than 1 cm.sup.2, exhibiting useful semiconductor properties have thus been difficult to attain. More importantly, the obtained mobilities, even over small areas, have often been unacceptably low or non-uniform compared to films formed by vapor phase techniques. (See, e.g., A. Stabel and J. P. Rabe, "Scanning tunneling microscopy of alkylated oligothiophenes at interfaces with graphite," Synthetic Metals, Vol. 67, 47 (1994); H. E. Katz et al., "Synthesis, Solubility, and Field-Effect Mobility of Elongated and Oxa-substituted .alpha.,.omega.-Dialkyl Thiophene Oligomers. Extension of `Polar Intermediate` Synthetic Strategy and Solution Deposition on Transistor Substrates," Chemistry of Materials, Vol. 10, No. 2, 633 (1998); and H. Akimichi et al., "Field-effect transistors using alkyl substituted oligothiophenes," Appl. Phys. Lett., Vol. 58, No. 14, 1500 (1991).)
Spin-coating of oligomeric solutions onto a substrate has been performed more successfully, as reflected in C. D. Dimitrakopoulos et al., "trans-trans-2,5-Bis-[2-{5-(2,2'-bithienyl)} ethenyl]thiophene: synthesis, characterization, thin film deposition and fabrication of organic field-effect "transistors," Synthetic Metals, Vol. 89, 193 (1997); and F. Garnier et al., "Dihexylquarterthiophene, A Two-Dimensional Liquid Crystal-like Organic Semiconductor with High Transport Properties," Chem. Mater., Vol. 10, 3334 (1998). Spin-coating is wasteful, however, in that much of the solution flies off the substrate. Also, the technique is incompatible with the desired printing fabrication processes for organic TFTs, e.g., reel-to-reel processes on a flexible substrate, and is therefore more of a laboratory technique than a potential commercial process.
Thus, improved methods are desired for forming organic semiconductor films from low molecular weight organic semiconductor compounds. Of particular interest are non-spinning techniques for depositing uniform, ordered organic films from solution.